The proposed research is to elucidate the physiological mechanisms involved in swelling of the blastocyst, which is an important pre-requisite for implantation. This process is intimately connected with the coupled transport of ions and water across the trophectoderm. The nature of the mechanisms involved necessitates the investigation of the short circuit current across this epithelium. The technique will be used to analyze the ionic basis of the short-circuit current, the importance of the paracellular pathway and the mode of action of drugs on transport of ions across the trophectoderm. Particular emphasis will be placed on prostaglandin E2 since anatgonists of this substance have been shown to prevent swelling of the blastocyst, and a reduction in implantation rate when instilled into the uterus. Parallel studies will also be done on the capability of the blastocyst to synthesize endogenous prostablandins from fatty acid precursors. Studies of the development of ouabain-sensitive and amiloride-sensitive transport systems in preimplantation embryos will be studied by cytochemical and biochemical means to provide one of the few descriptions at the molecular level of the formation of a functional epithelium.